Convergence correction devices serve for the correction of parameters in the raster deflection, for example the correction of north/south and east/west distortions, of pin-cushion distortions, of nonlinearities in the deflection, and other geometric distortions in the horizontal or vertical directions. A particular field of application is convergence correction in a television projection receiver in which the pictures of three monochromatic picture tubes are projected onto a picture screen. The television receiver is understood as any receiver in which pictures are reproduced electronically or using a raster. The receiver can be fed from an arbitrary video signal source with a television broadcast signal, or as a pure monitor with an RGB signal, an FBAS signal or, separately, with a luminance signal and a colour carrier.
The correction of the deflection parameters is undertaken with the aid of correction values which are stored in a convergence circuit. The stored correction values are converted in a digital-to-analogue converter into an analogue control signal and fed to a driver circuit consisting of a pre-amplifier and an output amplifier. This driver circuit outputs a current, corresponding to the correction value, to a correction coil. The magnitude of the correction current also depends on the gain of the driver circuit, which can vary during operation, in particular owing to temperature effects.